‘HE was in his own little world,” someone says of Henry Darger, the subject of the documentary “In the Realms of the Unreal.”

Darger was a rather strange individual, a janitor at a Catholic girls’ school who lived for 43 years in a second-floor room on Chicago’s North Side.

Although his life was quite simple – he “was almost like a homeless, but he wasn’t homeless, just poor,” we’re told in the film, written and directed by Jessica Yu – his imagination was exciting and sexually provocative.

He died in 1973, an 81-year-old recluse, leaving behind 300 paintings (mostly of little girls, many naked), thousands of pages of notes and journals, and a 15,000-page, illustrated novel called “The Realms of the Unreal,” built around a child-slave revolution in another world.

Using Darger’s own drawings (some of them animated by the filmmakers) and words, as well as interviews with the few people who came in contact with him, Yu presents a compelling, somewhat disturbing portrait of the artist, who in 2000 was the subject of a major exhibit that toured the world.

Running time: 81 minutes. Not rated (disturbing drawings). At Film Forum, Houston Street, west of Sixth Avenue. Through Jan. 4.